This invention relates to an FM stereo receiver, and more particularly to an automatic noise reduction system of the FM stereo receiver.
In an FM stereo receiver, upon decrease in the antenna input level and carrier frequency deviation of an FM stereo transmission signal, the signal-to-noise ratio of reproduced stereo signals is sometimes decreased to interrupt high quality reproduction of stereo signals. Most of the noise components generated upon reception and demodulation of the FM stereo transmission signal are triangular noises contained in opposite phase relationship, in the right and left audio outputs of a stereo demodulator. Further, human's ears are more sensitive to high frequency noise components than low frequency noise components.
For the above-mentioned reasons, a conventional FM stereo receiver is provided with means called a multiplex noise filter or high blend circuit and designed to operate mostly by a manual switch to blend with each other high frequency components of the signals appearing on the right and left output channels, thereby to cancel high frequency components of the triangular noises upon decrease in the SN ratio. However, such high blend circuit included in a conventional receiver is impotent over the mutually opposite phase components of intermediate and lower frequency ranges, the noise components contained in the same phase relationship in both output channels, and the high frequency noise components contained in only either one of the output channels.
When, in the case of using the above-mentioned manually operable high blend circuit, it is erroneously placed in an operative condition with the SN ratio kept to be good, the channel separation is unnecessarily decreased. Further, if, in the case of manual operation, the SN ratio is once decreased during the recording of stereo signals, extreme inconveniences will result.
The antenna input level is not always kept constant but fluctuates from time to time owing to weather, etc. and besides the frequency deviation of carrier signal also fluctuates from instant to instant during the period of the same program, for which reason there is a possibility of the SN ratio being often varied.